1. Field of the Invention
The present invention relates to a technique for detecting a movement of an object by using image processing.
2. Description of the Related Art
When printing is performed while a medium such as a print sheet is being conveyed, if conveyance accuracy is low, density unevenness of a halftone image or a magnification error may be generated, thereby deteriorating quality of acquired print images. Therefore, although high-performance components are adopted and an accurate conveyance mechanism is mounted, requests about print qualities are demanding, and further enhancement of accuracy is requested. In addition, requests about costs are also demanding. Both of high accuracy and low costs are requested.
To address these issues, and thus, to detect a movement of a medium with high accuracy and perform stable conveyance by a feedback control, it has been attempted to capture the image of a surface of the medium and detect the movement of the medium that is being conveyed by image processing.
Japanese Patent Application Laid-Open No. 2007-217176 discusses a method for detecting the movement of the medium. According to Japanese Patent Application Laid-Open No. 2007-217176, an image sensor captures images of a surface of a moving medium several times in chronological order, the acquired images are compared with each other by performing pattern matching processing, and thus an amount of the movement of the medium can be detected. Hereinafter, a method in which a movement state is detected by directly detecting the surface of the object is referred to as “direct sensing”, and a detector using this method is referred to as a “direct sensor”.
When the direct sensing is used to detect the movement, the surface of the medium is to be optically and sufficiently identified and unique patterns are to be obvious. However an applicant of the present exemplary embodiment has found that, under conditions described below, accuracy of pattern matching can be deteriorated.
When the object moves while being captured, the image sensor captures the images having objet blur. If the image sensor captures two images moving at the same speed at different times, both images have the similar object blur. However, since amounts of the object blurs have no relative difference therebetwen, unless the amounts of the object blurs are large enough to delete unique image patterns, no serious incidents with accuracy of the pattern matching arise.
The situation can arise when the object moves at largely different speeds, when the images are captured, to cause the object blurs having largely different object blur widths therebetween. For example, in FIG. 15, a first image 912 has an object blur width 921, a second image 913 has an object blur width 922. A relative difference 920 indicates an amount of difference between the object blur widths. The larger the relative difference 920 is, the more deterioration the accuracy of the pattern matching has.